1. Field of Invention
The invention relates to the fabrication of wires containing radioactive californium-252. In particular, thinner radioactive wires having a more nearly circular cross section are achieved through a series of pressurization steps using a collet fixture used as a wire shaping apparatus.
2. Description of Prior Art
Brachytherapy is a method of treating cancer in which a radioactive source is positioned within the body at the site of the tumor. Neutron brachytherapy using californium-252 (Cf-252) radioactive sources has been practiced for 30 years, typically using either low dose rate (LDR) treatments or, for some tumors, high dose rate (HDR) therapy. HDR therapy is preferred because of its shorter treatment times. However, to date, only relatively large (approximately 3-mm outer diameter) sources contain sufficient Cf-252 mass to provide neutron intensities in the HDR range. The present HDR sources are too large to be used for interstitial (intraorgan) treatments, but are ideal for intracavitary treatments (gynecological, rectal, head, neck, and oral cavity treatments, etc.). As a result, interstitial treatments have been limited to the LDR regime.
High specific activity (HSA) Cf-252 source material has not previously been available in a form thin enough for use in smaller sources of appropriate sizes for interstitial treatments (e.g., brain, prostate, breast, lung, etc.). The present invention seeks to overcome this limitation by providing thinner high specific activity Cf-252 source material.
Oak Ridge National Laboratory (ORNL) is the only production source for Cf-252 in the United States. One other production center is in Russia. Both sites manufacture Cf-252 medical sources. Russia's Cf-252 source forms do not have a specific activity as high as that available from the Cf-252 cermet wire forms available from ORNL.
The fabrication of very thin gamma or photon-emitting source wires for use in brachytherapy sources is typically not as difficult as that of Cf-containing cermet wire. Conventional gamma sources have a great advantage in that very thin wires can first be fabricated with nonradioactive material, and then placed inside the core of a nuclear reactor where they are activated to the desired radioactivity. Segments of these activated wires are then sealed as radioactive sources and used as is. The use of iridium-192 wires as small as 0.0134″ in brachytherapy sources has been reported.
Unfortunately, the man-made element californium cannot be handled outside of heavily shielded containments such as hot cells. Californium-252 is obtained through a wet chemical process of dissolutions, purifications, precipitations, and wire fabrication inside a heavily shielded and highly contaminated hot cell. All operations must be performed remotely, and hands-on operations with the resultant cermet wires are not possible due to intense neutron emission. The nature of cermet wire, as a metallic matrix with ceramic impurities (californium oxide), makes miniaturization increasingly difficult in the production of a structurally sound wire, especially a heavily loaded (>1% by weight Cf-252) cermet wire.
ORNL's cermet wires contain californium oxide blended within a palladium metal matrix. The ceramic oxide acts as an impurity within the palladium, and degrades Pd workability as the oxide concentration increases above 1% by weight. The wires are formed by rolling a previously melted cermet pellet through smaller and smaller grooves of a jeweler's rolling mill. The rolling mill currently used within the Californium Facility hot cells can produce a wire with measurable dimensions approaching 0.75 mm. However, the effective diameter is closer to 1 mm due to its trapezoidal cross-section.
An example of prior technology is D. S. Erickson and A. Feiring, “Guidewire Steering Handle and Method of Using Same”, U.S. Pat. No. 5,755,695, Issued May 26, 1998.
We have developed an effective method for significantly reducing the cross section of ORNL's heavily loaded (>1% by weight Cf-252) cermet wires. Of particular advantage is that our method is well suited for use in hot cells where the high-intensity, miniature Cf-252 sources are produced.